Process for preparing furfural from biomass

ABSTRACT

Furfural is produced from biomass material containing pentosan, in high yields, in a production process comprising treating the biomass with a solution containing at least one α-hydroxysulfonic acid thereby hydrolyzing the biomass to produce a product stream containing at least one C 5 -carbohydrate compound in monomeric and/or oligomeric form, and dehydrating the C 5 -carbohydrate compound in the presence of a heterogeneous solid acid catalyst, in a biphasic reaction medium comprising an aqueous phase and a water-immiscible organic phase, at a temperature in the range of from about 100° C. to about 250° C. to produce a dehydration product stream containing furfural. An aqueous stream is separated from the dehydration product, which can be optionally recycled to the hydrolysis step.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 62/037,195 filed Aug. 14, 2014, the entire disclosure of which ishereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a process for preparing furfural from biomass,and more specifically to a treatment of biomass and production offurfural from materials containing polysaccharides and/orlignocelluloses.

BACKGROUND OF THE INVENTION

Lignocellulosic biomass is viewed as an abundant renewable resource forchemicals due to the presence of sugars in the cell walls of plants.More than 50% of the organic carbon on the earth's surface is containedin plants. This lignocellulosic biomass is comprised of hemicelluloses,cellulose and smaller portions of lignin and protein. These structuralcomponents are comprised primarily of pentose and hexose sugarsmonomers. Cellulose is a polymer comprised mostly of condensationpolymerized glucose and hemicellulose is a precursor to pentose sugars,mostly xylose. These sugars can be converted into valuable components,provided they can be liberated from the cell walls and polymers thatcontain them. However, plant cell walls have evolved considerableresistance to microbial, mechanical or chemical breakdown to yieldcomponent sugars. In order to overcome recalcitrance ground biomass isaltered by a chemical process known as pretreatment. The aim of thepretreatment is to hydrolyze the hemicellulose, break down theprotective lignin structure and disrupt the crystalline structure ofcellulose. All of these steps enhance enzymatic accessibility to thecellulose during the subsequent hydrolysis (saccharification) step.

The original approaches dating back to the early 19th century involvecomplete chemical hydrolysis using concentrated mineral acids such ashydrochloric acid, nitric, or sulfuric acid. Numerous improvements tothese processes have been made earning higher sugar yields from thebiomass feedstock. These higher acid concentration approaches providehigher yields of sugars, but due to economic and environmental reasonsthe acids must be recovered. The primary obstacle to practicing thisform of saccharification has been the challenges associated withrecovery of the acid [M. Galbe and G. Zacchi, Appl. Microbiol.Biotechnol. Vol. 59, pp. 618-628 (2002)]. Recent efforts towardseparating sulfuric acid and sugars using ion resin separation orhydrochloric acid and sugars via an amine extraction process andsubsequent thermal regeneration of the acid have been described in U.S.Pat. No. 5,820,687. However, both of these approaches are cumbersome andexpensive in practice.

Dilute acid processes have also been attempted to perform chemicalsaccharification and one such example is the Scholler-Tornesch Process.However usage of dilute acid requires higher temperatures and thisusually results in low yields of the desired sugars due to thermaldegradation of the monosaccharides. Numerous approaches of this typehave been made in the past and all have failed to meet economic hurdles.[See, for example, Lim Koon Ong, “Conversion of Lignocellulosic Biomassto Fuel Ethanol—A Brief Review,” The Planter, Vol. 80, No. 941, August2004, and, “Cell Wall Saccharification,” Ralf Moller, in Outputs fromthe EPOBIO Project, 2006; Published by CPL Press, Tall Gables, TheSydings, Speen, Newbury, Berks RG14 1RZ, UK].

The saccharification of the cellulose enzymatically holds promise ofgreater yields of sugars under milder conditions and is thereforeconsidered by many to be more economically attractive. The recalcitranceof the raw biomass to enzymatic hydrolysis necessitates a pretreatmentto enhance the susceptibility of the cellulose to hydrolytic enzymes. Anumber of pretreatment methods, such as described by Mosier, et al.[Bioresource Technology, Vol. 96, pp. 673-686 (2005)], have beendeveloped to alter the structural and chemical composition of biomass toimprove enzymatic conversion. Such methods include treatment with adilute acid steam explosion, as described in U.S. Pat. No. 4,461,648,hydrothermal pretreatment without the addition of chemicals as describedin WO 2007/009463 A2, ammonia freeze explosion process as described byHoltzapple, M. T., et al. [Applied Biochemistry and Biotechnology,28/29, pp. 59-74], and an organosolve extraction process described inU.S. Pat. No. 4,409,032. Despite these approaches, such pretreatment hasbeen cited as the most expensive process in biomass-to-fuels conversion[Ind. Eng. Chem. Res., Vol. 48(8), 3713-3729.(2009)].

One pretreatment that has been extensively explored is a hightemperature, dilute-sulfuric acid (H₂SO₄) process, which effectivelyhydrolyzes the hemicellulosic portion of the biomass to soluble sugarsand exposes the cellulose so that enzymatic Saccharification issuccessful. The parameters which can be employed to control theconditions of the pretreatment are time, temperature, and acid loading.These are often combined in a mathematical equation termed the combinedseverity factor. In general, the higher the acid loading employed, thelower the temperature that can be employed; this comes at a cost of acidand its need to recycle the acid. Conversely, the lower the temperature,the longer the pretreatment process takes; this comes at the cost ofproductivity. However the use of the higher concentrations of acidrequired to lower the pretreatment temperatures below that wherefurfural formation becomes facile [B. P. Lavarack, et al., Biomass andBioenergy, Vol. 23, pp. 367-380 (2002)] once again requires the recoveryof the strong acid. If dilute acid streams and higher temperatures areemployed the pretreatment reaction the acid passing downstream to theenzymatic hydrolysis and subsequent fermentation steps must beneutralized resulting in inorganic salts which complicates downstreamprocessing and requires more expensive waste water treatment systems.This also results in increased chemical costs for acid and baseconsumption.

More recently, in US20120122152, α-hydroxysulfonic acids have been shownto be effective in the pretreatment and hydrolysis of biomass with theadditional benefit of being recoverable and recyclable through reversalto the acids primary components (aldehyde, SO₂ and water). Thispretreatment process has been shown to provide numerous benefitscompared to dilute mineral acid pretreatment. However, at the lowtemperature, the formation of furfural is low.

A method for preparing furfural may use a batch process based on aQuaker Oats technology developed in 1920 using sulfuric acid. The batchprocess is known to be significantly inefficient. That is, thetheoretical furfural yield is about 30 to 40%, the residence time in thereactor is significant long as 4.5 to 5.5 hours, water of 50 MT per 1 MTof furfural is consumed, and a significant amount of harmful substanceis included in effluents. In addition, costs consumed for working areconsiderably increased

Further, whether batch or continuous, when using such acid catalyst, theprocess corrosion and the acid wastes are generated, such that it isdifficult to separate, recover, and recycle a non-reactive raw materialand the acid catalyst. Further, the economic efficiency of the processmay be very vulnerable according to the increase in investment costs ofprocess facility and low product yield and environmental toxicity,recovery, and recycle may be complicated even in the process of using anorganic solvent

SUMMARY OF THE INVENTION

The inventions disclosed and taught herein are directed to methods forthe synthesis of furfural and similar organic materials from a biomassfeedstock in high yields that optionally allows for easier recycle.

In an embodiment of the present invention, a method is provided forproducing furfural from biomass material containing pentosan:

(a) providing a biomass containing pentosan;(b) contacting the biomass with a solution containing at least oneα-hydroxysulfonic acid thereby hydrolyzing the biomass to produce aproduct stream containing at least one C₅-carbohydrate compound inmonomeric and/or oligomeric form, and α-hydroxysulfonic acid;(c) separating at least a portion of the α-hydroxysulfonic acid from theproduct stream containing at least one C₅-carbohydrate compound toprovide an acid-removed product stream containing the at least oneC₅-carbohydrate compound and recovering the α-hydroxysulfonic acid inits component form;(d) separating a liquid stream containing said at least oneC₅-carbohydrate compound and a wet solid stream containing remainingbiomass from the acid-removed product;(e) dehydrating the C₅-carbohydrate compound in at least a first portionof the liquid stream in the presence of a heterogenous solid acidcatalyst, in a biphasic reaction medium comprising an aqueous phase anda water-immiscible organic phase, at a temperature in the range of fromabout 100° C. to about 250° C.;(f) separating an organic phase stream containing furfural and anaqueous stream from the dehydration product stream;(g) recycling at least a portion of the aqueous stream or a secondportion of the liquid stream to step (b);(h) recovering furfural from the organic phase stream.

The features and advantages of the invention will be apparent to thoseskilled in the art. While numerous changes may be made by those skilledin the art, such changes are within the spirit of the invention.

BRIEF DESCRIPTION OF THE DRAWING

This drawing illustrates certain aspects of some of the embodiments ofthe invention, and should not be used to limit or define the invention.

The FIGURE schematically illustrates a block flow diagram of anembodiment of the furfural production process of the invention frombiomass.

DETAILED DESCRIPTION OF THE INVENTION

It has been found that the present invention provides an improved methodfor the production of furfural from biomass in a batch, continuous orsemi-continuous manner, (optionally as a closed-loop process). By use ofthe α-hydroxysulfonic acid, the acid can be readily separated by heatingor reducing pressure and recycled, and only require a fraction of anacid and fraction of a time compared to conventional process todehydrate the C₅-carbohydrate compound, thus increasing efficiency anddecreasing complications. By use of the α-hydroxysulfonic acid forpretreatment of biomass to produce a liquid stream containingC₅-carbohydrate compound, it is possible to use a solid acid catalystfor subsequent dehydration of the C₅-carbohydrate compound, increasingefficiency and decreasing complications in the system. Further, byseparating a liquid stream containing C₅-carbohydrate compound from thewet solids from the product stream from the α-hydroxysulfonic acidpre-treatment step, it has been found that furfural can be producedwithout excessive degradation of C6 sugars in the subsequent dehydrationstep. For example, the controlled return of the aqueous stream followingthe dehydration of the C5 carbohydrates extracted from the biomassallows for maintaining an optimized reaction process flow. Additionally,the method allows for increased amounts of both C5-carbohydrate andC6-carbohydrate-containing intermediate product streams to beefficiently separated and recovered and sent on to further upgradingand/or purification steps (dehydration, fermentation, etc), whereasoften these intermediate products are lost or destroyed during treatmentsteps. Furthermore, the process methods allow for higher concentrationsof pentosan-comprising biomass to be treated, which increased theproduct concentration, thereby reducing the size of equipment andfacilitating the recovery of valuable intermediates and productsoverall. In addition, the use of extraction methods within the processallows for recovery of the desired product (furfural) without having todistill or strip much water with it (as azeotrope). When carried out insitu, for example during dehydration, it reduces the formation ofundesired by products such as humins and/or impurities and therebyincrease yield of desired product. Further, in an embodiment where theaqueous stream is recycled to the hydrolysis step, such recycle of theaqueous stream may be partial.

In a preferred embodiment, it has further been found that by titratingthe α-hydroxysulfonic acid salt with strong acid and then reverting theα-hydroxysulfonic acid as its primary components, the acid componentscan be recovered virtually quantitatively providing for a cost reductionin the reversible acid pretreatment process. When α-hydroxysulfonic acidencounters a basic species, such as a carbonate, the anionic salt formof the acid is generated. This acid salt is not reversible as theα-hydroxysulfonic acid must be in the protonic form to revert to primarycomponents. Since biomass is always accompanied by caustic inorganicmaterials, we have found that the formation of the anion salt ofα-hydroxysulfonic acid represent the largest “loss” of theα-hydroxysulfonic acid in the potential reversible acid pretreatmentprocess. It has been further found that a small amount of a mineral acidcan conveniently be used to also titrate the α-hydroxysulfonic acid saltand enhance recovery of the α-hydroxysulfonic acid by reverting its saltto the acid form and then recovering the α-hydroxysulfonic acid in itsprimary components. If the α-hydroxysulfonic acid cannot be recycled, itis expensive relative to mineral acids. Thus, by recovering theα-hydroxysulfonic acid from its acid salt, provides for a cost reductionin the treatment process.

The α-hydroxysulfonic acid is effective for treatment of biomasshydrolyzing the biomass to fermentable sugars like pentose such asxylose at lower temperature, (e.g., about 100° C. for α-hydroxymethanesulfonic acid or α-hydroxyethane sulfonic acid) producing littlefurfural in the process. A portion of the cellulose has also been shownto hydrolyze under these comparatively mild conditions. Otherpolysaccharides such as starch are also readily hydrolyzed to componentsugars by α-hydroxysulfonic acids. Further, the α-hydroxysulfonic acidis reversible to readily removable and recyclable materials unlikemineral acids such as sulfuric, phosphoric, or hydrochloric acid. Thelower temperatures and pressures employed in the biomass treatment leadsto lower equipment cost. The ability to recycle fragile pentose sugarsfrom the end of pretreatment to the inlet of pretreatment, without theirsubsequent conversion to undesirable materials such as furfural, allowslower consistencies in the pretreatment reaction itself, yet stillpassing a high consistency solids mixture containing high soluble sugarsout of pretreatment. Biomass pretreated in this manner has been shown tobe highly susceptible to additional saccharification, especially enzymemediated saccharification.

The α-hydroxysulfonic acids have the general formula

wherein R₁ and R₂ are individually hydrogen or hydrocarbyl with up toabout 9 carbon atoms that may or may not contain oxygen can be used inthe treatment of the instant invention. The alpha-hydroxysulfonic acidcan be a mixture of the aforementioned acids. The acid can generally beprepared by reacting at least one carbonyl compound or precursor ofcarbonyl compound (e.g., trioxane and paraformaldehyde) with sulfurdioxide or precursor of sulfur dioxide (e.g., sulfur and oxidant, orsulfur trioxide and reducing agent) and water according to the followinggeneral equation 1.

where R₁ and R₂ are individually hydrogen or hydrocarbyl with up toabout 9 carbon atoms or a mixture thereof.

Illustrative examples of carbonyl compounds useful to prepare thealpha-hydroxysulfonic acids used in this invention are found where

R₁═R₂═H (formaldehyde)R₁═H, R₂═CH₃ (acetaldehyde)R₁═H, R₂═CH₂CH₃ (propionaldehyde)R₁═H, R₂═CH₂CH₂CH₃ (n-butyraldehyde)R₁═H, R₂═CH(CH₃)₂ (i-butyraldehyde)R₁═H, R₂═CH₂OH (glycolaldehyde)R₁═H, R₂═CHOHCH₂OH (glyceraldehdye)R1═H, R2═C(═O)H (glyoxal)

R₁═R₂═CH₃ (acetone)R₁═CH₂OH, R₂═CH₃ (acetol)R₁═CH₃, R₂═CH₂CH₃ (methyl ethyl ketone)R₁═CH₃, R₂═CHC(CH₃)₂ (mesityl oxide)R₁═CH₃, R₂═CH₂CH(CH₃)₂ (methyl i-butyl ketone)R₁, R₂═(CH₂)₅ (cyclohexanone) orR₁═CH₃, R₂═CH₂Cl (chloroacetone)

The carbonyl compounds and its precursors can be a mixture of compoundsdescribed above. For example, the mixture can be a carbonyl compound ora precursor such as, for example, trioxane which is known to thermallyrevert to formaldehyde at elevated temperatures, metaldehdye which isknown to thermally revert to acetaldehyde at elevated temperatures, oran alcohol that maybe converted to the aldehyde by dehydrogenation ofthe alcohol to an aldehyde by any known methods. An example of such aconversion to aldehyde from alcohol is described below. An example of asource of carbonyl compounds maybe a mixture of hydroxyacetaldehyde andother aldehydes and ketones produced from fast pyrolysis oil such asdescribed in “Fast Pyrolysis and Bio-oil Upgrading, Biomass-to-DieselWorkshop”, Pacific Northwest National Laboratory, Richland, Wash., Sep.5-6, 2006. The carbonyl compounds and its precursors can also be amixture of ketones and/or aldehydes with or without alcohols that may beconverted to ketones and/or aldehydes, preferably in the range of 1 to 7carbon atoms.

The preparation of α-hydroxysulfonic acids by the combination of anorganic carbonyl compounds, SO₂ and water is a general reaction and isillustrated in equation 2 for acetone.

The α-hydroxysulfonic acids appear to be as strong as, if not strongerthan, HCl since an aqueous solution of the adduct has been reported toreact with NaCl freeing the weaker acid, HCl (see U.S. Pat. No.3,549,319).

The reaction in equation 1 is a true equilibrium, which results infacile reversibility of the acid. That is, when heated, the equilibriumshifts towards the starting carbonyl, sulfur dioxide, and water(component form). If the volatile components (e.g. sulfur dioxide) isallowed to depart the reaction mixture via vaporization or othermethods, the acid reaction completely reverses and the solution becomeseffectively neutral. Thus, by increasing the temperature and/or loweringthe pressure, the sulfur dioxide can be driven off and the reactioncompletely reverses due to Le Châteliers principle, the fate of thecarbonyl compound is dependant upon the nature of the material employed.If the carbonyl is also volatile (e.g. acetaldehyde), this material isalso easily removed in the vapor phase. Carbonyl compounds such asbenzaldehyde, which are sparingly soluble in water, can form a secondorganic phase and be separated by mechanical means. Thus, the carbonylcan be removed by conventional means, e.g., continued application ofheat and/or vacuum, steam and nitrogen stripping, solvent washing,centrifugation, etc. Therefore, the formation of these acids isreversible in that as the temperature is raised, the sulfur dioxideand/or aldehyde and/or ketone can be flashed from the mixture andcondensed or absorbed elsewhere in order to be recycled. Thesereversible acids, which are approximately as strong as strong mineralacids, are effective in biomass treatment reactions.

Since the acids are effectively removed from the reaction mixturefollowing treatment, neutralization with base to complicate downstreamprocessing is substantially avoided. The ability to reverse and recyclethese acids also allows the use of higher concentrations than wouldotherwise be economically or environmentally practical. As a directresult, the temperature employed in biomass treatment can be reduced todiminish the formation of byproducts such as furfural orhydroxymethylfurfural.

It has been found that the position of the equilibrium given in equation1 at any given temperature and pressure is highly influenced by thenature of the carbonyl compound employed, steric and electronic effectshaving a strong influence on the thermal stability of the acid. Moresteric bulk around the carbonyl tending to favor a lower thermalstability of the acid form. Thus, one can tune the strength of the acidand the temperature of facile decomposition by the selection of theappropriate carbonyl compound.

In one embodiment, the acetaldehyde starting material to produce thealpha-hydroxysulfonic acids can be provided by converting ethanol,produced from the fermentation of the treated biomass of the inventionprocess, to acetaldehyde by dehydrogenation or oxidation. Such processesare described in US20130196400 which disclosure is herein incorporatedby reference in its entirety.

As used herein, the term “biomass” means organic materials produced byplants (e.g., leaves, roots, seeds and stalks). Common sources ofbiomass include: agricultural wastes (e.g., corn stalks, straw, seedhulls, sugarcane leavings, bagasse, nutshells, and manure from cattle,poultry, and hogs); wood materials (e.g., wood or bark, sawdust, timberslash, and mill scrap); municipal waste (e.g., waste paper and yardclippings); and energy crops (e.g., poplars, willows, switch grass,alfalfa, prairie bluestream, corn, soybean, algae and seaweed). The term“biomass” also refers to the primary building blocks of all the above,including, but not limited to, saccharides, lignins, celluloses,hemicelluloses, and starches. The term “polysaccharides” refers topolymeric carbohydrate structures, of repeating units (either mono- ordi-saccharides) joined together by glycosidic bonds. These structuresare often linear, but may contain various degrees of branching.

Examples include storage polysaccharides such as starch and glycogen,and structural polysaccharides such as cellulose and chitin.

As used herein the term “pentosan” refers to a polysaccharide containingC5 carbohydrate monomeric unit.

As used herein, the term “carbohydrate” is defined as a compound thatconsists only of carbon, hydrogen, and oxygen atoms, wherein the ratioof carbon atoms, hydrogen atoms, to oxygen atoms when converted tomonomeric sugars upon hydrolysis is 1:2:1. Well known examples ofcarbohydrates include sugars and sugar-derived oligomers andsugar-derived polymers. The term “C5 carbohydrate(s)” refers to anycarbohydrate, without limitation, that has five (5) carbon atoms in itsmonomeric unit. The definition includes pentose sugars of anydescription and stereoisomerism (e.g., D/L aldopentoses and D/Lketopentoses). C5 carbohydrates can include (by way of example and notlimitation) xylose, arabinose, lyxose, ribose, ribulose, and xylulose,in their monomeric and polymeric forms. Polymeric C5 carbonydrates cancontain several C5 carbohydrate monomers and in some instances evencontain some (lesser) amount of C6 carbohydrate monomers. According tothe invention, the term “pentose”, in addition to chemical compounds offormula C5H10O5 ring such as xylose or arabinose or mixtures thereof,may also include derivatives products including pentose and theirderivatives. The term “C6 carbohydrate” refers to any carbohydrate,without limitation, that has six (6) carbon atoms in its monomeric unit.The definition includes hexose sugars of any description andstereoisomerism (e.g., D/L aldohexoses and D/L ketohexoses). C6carbohydrates include (by way of example and not limitation) allose,altrose, fructose, galactose, glucose, gulose, idose, mannose, psicose,sorbose, tagatose, and talose, in their monomeric, oligomeric andpolymeric forms. Polymeric C6 carbohydrates can contain several C6carbohydrate monomers, and in some instances even contain some (lesser)amount of C5 carbohydrate monomers.

The term “dehydration”, as used herein, refers to the removal of a watermolecule from a molecule that contains at least one hydroxyl group.

As used herein, the term “humins” refers to the dark, amorphous andundesirable acid byproducts and resinous material resulting from sugarand other organic compound degradation. Humins may also be produced byacid hydrolysis of carbohydrates. Yang and Sen [Chem. Sus. Chem., Vol.3, pp. 597-603 (2010)] report the formation of humins during productionof fuels from carbohydrates such as fructose, and speculate that thehumins are formed by acid-catalyzed dehydration. The molecular weight ofheavy components of humins can range from 2.5 to 30 kDa.

As used herein, the term “miscible” refers to a mixture of componentsthat, when combined, form a single phase (i.e., the mixture is“monophasic”) under specified conditions (e.g., componentconcentrations, temperature).

As used herein, the term “immiscible” refers to a mixture of componentsthat, when combined, form a two, or more, phases under specifiedconditions (e.g., component concentrations, temperature).

As used herein, the term “monophasic” refers to a reaction medium thatincludes only one liquid phase. Some examples are water, aqueoussolutions, and solutions containing aqueous and organic solvents thatare miscible with each other. The term “monophasic” can also be used todescribe a method employing such a reaction medium.

As used herein, the term “biphasic” refers to a reaction medium thatincludes two immiscible liquid phases, for example, an aqueous phase anda water-immiscible organic solvent phase. The term “biphasic” can alsobe used to describe a method employing such a reaction medium.

The FIGURE shows an embodiment of the present invention for the improvedproduction of furfural from biomass. In this embodiment, a biomassfeedstock containing pentosan (“pentosan-containing biomass feedstock”)112 is introduced to a hydrolysis reaction system 114 along with anoptional recycle stream 218 and optional recycle stream 318. Thehydrolysis reaction system 114 may comprise a number of componentsincluding in situ generated α-hydroxysulfonic acid. The term “in situ”as used herein refers to a component that is produced within the overallprocess; it is not limited to a particular reactor for production or useand is therefore synonymous with an in process generated component. Thehydrolysis reaction system 114 can contain one or more reactors andoptionally solids or slurry extractors. The reacted product stream 116,containing at least one C5-carbohydrate, at least one α-hydroxysulfonicacid, and optionally at least one salt of α-hydroxysulfonic acid andsolids comprising lignin, cellulose and hemicellulosic material isintroduced to acid removal system 120 where the α-hydroxysulfonic acidis removed in its component form, then is recovered 122 (and optionallyscrubbed 124), and produces a product stream 126. The recovered acids(whether in acid form or component form) are recycled via stream 118 tothe hydrolysis reaction system 114. The product stream 126 contains atleast one C5-carbohydrate, optionally C6-carbohydrate, substantiallywithout the alpha-hydroxysulfonic acids. Optionally, at least a portionof the liquid on product stream 116 containing α-hydroxysulfonic acidcan be recycled to the hydrolysis reaction system 114 (not shown).

The second product stream 126 is provided to a separation system 200where a high solids/liquid mixture (“wet solids”) can be separated fromthe acid-removed product stream to form a wet solids stream 220containing undissolved solids containing cellulose, and a bulk liquidstream 210 that may constitute up to 20 to 95 wt % of the liquid fromthe acid-removed product stream that contains C5-carbohydrates (pentose)and optionally hexose and optionally the salt of α-hydroxysulfonicacids. In one embodiment, the wet solids stream containing cellulose mayfurther be hydrolyzed by other methods, for example by enzymes tofurther hydrolyze the biomass to sugar products containing hexose (e.g.,glucose) and fermented to produce alcohols and acids such as disclosedin US Publication Nos. 2009/0061490, 2012/0122152, 2013/0295629, andU.S. Pat. No. 7,781,191 which disclosures are hereby incorporated byreference. In another embodiment, the wet solids stream can suitably beused to generate power by the burning of the wet solid residue e.g. in aco-generation boiler. Alternatively, the wet solid product stream may beconverted and optionally dried to form pellets, which can be used toproduce for instance power at remote locations.

At least a portion (a second portion) of the bulk liquid stream 210 maybe optionally recycled to the hydrolysis reaction system via 218 wherethe bulk liquid stream comprise greater than about 2 wt %, preferably 5wt % or greater, more preferably about 8 wt % or greater, of C5carbohydrates and C6 Carbohydrates based on the bulk liquid stream. Thebulk liquid stream is preferably recycled in such a manner as to keepthe hydrolysis reaction pumpable, preferably about 20 wt % or less ofsolids content in the hydrolysis reactor and further accumulate the C5carbohydrates content of the bulk liquid stream 210 through recycle. Asone embodiment, a portion of the bulk liquid recycle stream 218 can beused to dilute the hydrolysis reaction system 114 towards the inlet ofthe biomass in the hydrolysis reactor in the system, and/or for ease ofsolids extraction at the reactor bottoms (or reactor system exit) or canbe added to a extractor or towards the reactor product stream 116 fordilution

A mineral acid 135 may be optionally introduced to 114 (or optionallyadded instead to 116, 120, 126, 210 or 218 which are not shown) in anamount sufficient to titrate salt of alpha-hydroxysulfonic acid if any.The alpha hydroxy-sulfonic acid may be optionally recovered in itscomponent form from titration then recovered (and optionally scrubbed)that may be recycled to the hydrolysis reaction system.

The dehydration step 300 occurs in a biphasic reaction medium (containsaqueous phase and water-immiscible organic phase), the aqueous phasebeing that carried through from separation system 200, the organic phasebeing one or more organic solvents that are substantially immisciblewith the aqueous phase. The use of organic solvent with preferredselectivity towards furfural extraction, extracts furfural from theaqueous phase as it is formed during the dehydration reaction. This mayimprove overall furfural yield. A further advantage is that byextracting the furfural into the organic phase, the undesired loss offurfural via degradation reactions happening in the aqueous phase isreduced.

Following the dehydration step 300, dehydration product stream 310 istransferred to a liquid-liquid extractor 330 for the extraction step,optionally after cooling of the stream. The extractor 330 can beoperated at a temperature range from about room temperature to about thedehydration temperature, so long as the liquid separates into two liquidphases at the extractor temperature. The organic phase is separated fromthe aqueous phase, and thus obtained aqueous recycle stream 318 at leastin part may optionally be fed directly back into the process loop at thehydrolysis reaction system 114. Depending upon the salt, and optionalother organic byproduct, content of the aqueous stream, aqueous recyclestream 318 may be treated to remove unwanted or excessive amounts ofsalts and/or organic byproducts. Preferably, aqueous recycle stream issubjected to a separation step (not shown). The recovered aqueousrecycle stream obtained after treatment of aqueous recycle stream, isreintroduced to the hydrolysis reaction system 114. Salts, andoptionally other organic byproducts like humins and acetic acids, areformed as a byproduct during one or more of the process steps.Typically, part of stream 318 may also be purged 360 from the process toprevent the build-up of byproducts.

Prior to undergoing the liquid-liquid extraction step 330, dehydrationproduct stream 310 may optionally be routed through a, preferablysolid/liquid, separation step, to remove any insoluble humins or othertar that may have been formed during the dehydration step 300, and whichmay otherwise negatively interfere with the separation of the organicphase from the aqueous phase, or later separation or purification steps400. The humins or tar will predominantly end up in the solid phase andwill thus not, or to a lesser extent, affect the subsequentorganic/aqueous separation step 330. Formation of tar, char, and/orhumins is a well known problem associated with the production ofbio-based products, and their non removal from the production stream canresult in problems during downstream purification and/or separationsteps.

The organic phase is recovered from extraction step 330 as organicproduct stream 350, containing the target organic compounds such asfurfural, furfural derivatives (such as hydroxyl methylfurfural (HMF),methyl-furfural) and levulinic acid. Although, part of organic productstream 350 may be recycled to dehydration step (or reactor(s)) 300, themajority of organic product stream 350 is subjected to a separationstep, preferably one or more distillation steps, in a recovery zone 400.If the extraction solvent is low-boiler, it will be removed as topproduct, eventually together with water, water/furfural azeotropicmixture and other light organic products such as acetic acid. Furfuralwill then be removed as bottom product of 400, optionally with otherhigh-boiling impurities such as HMF, levulinic acid or soluble humins.If the extractive solvent is high-boiler, it will be removed as bottomproduct of 400 together with other high-boiling impurities. Furfural isthen removed as top product, optionally with other low-boilingimpurities (AA) and optionally with water, e.g. as azeotropic mixture.Both top- and bottom stream of 400 may optionally undergo furtherpurification, e.g. by distillation, to remove undesired impurities fromsolvent or from furfural. Residual water from the reaction that was notremoved during the liquid-liquid extraction step, and which may containacetic acid or other low-boiling impurities, is removed from recoveryzone 400, with recovery of furfural via stream 420.

Organic solvents 410 removed/recovered during the separation in recoveryzone 400 may be recycled back into the process, such as byreintroduction back into the dehydration step 300 via a organic recyclestream 410, in order to minimize production costs and maintain thereaction process and process efficiency. Alternatively, at least part ofthe organic solvents can be directed to a further solvent purificationprocess such as column distillation/separation or solvent-solventextraction, prior to reintroduction back into the production process, soas to remove impurities, primarily humins (heavy byproducts), as well aspurify the solvent before reintroduction (not shown). After the solventpurification step, fresh solvent may be added to the purified solventstream or organic recycle stream 410 prior to reintroduction to thedehydration step 300 or introduced to the dehydration step 300 so as tomaintain the required volume of organic phase in the dehydration step.

In another embodiment, a biomass feedstock containing pentosan(“pentosan-containing biomass feedstock”) 112 is introduced to ahydrolysis reaction system 114 along with both the recycle stream 218and an aqueous recycle stream 318.

The biomass is typically preprocessed to suitable particles size thatmay include grinding. Not intending to restrict the scope of theinvention, it is typically found that it is easier to process smallerparticles of biomass. Biomass that is size reduced to facilitatehandling (e.g. less than 1.3 cm) are particularly susceptible materials.

Various factors affect the conversion of the biomass feedstock in thehydrolysis reaction. The carbonyl compound or incipient carbonylcompound (such as trioxane) with sulfur dioxide and water should beadded to in an amount and under conditions effective to formalpha-hydroxysulfonic acids. The temperature and pressure of thehydrolysis reaction should be in the range to form alpha-hydroxysulfonicacids and to hydrolyze biomass into fermentable sugars. The amount ofcarbonyl compound or its precursor and sulfur dioxide should be toproduce alpha-hydroxysulfonic acids in the range from about 1 wt %,preferably from about 5 wt %, to about 55 wt %, preferably to about 40wt %, more preferably to about 20 wt %, based on the total solution. Forthe reaction, excess sulfur dioxide is not necessary, but any excesssulfur dioxide may be used to drive the equilibrium in eq. 1 to favorthe acid form at elevated temperatures. The contacting conditions of thehydrolysis reaction may be conducted at temperatures preferably at leastfrom about 50° C. depending on the alpha-hydroxysulfonic acid used,although such temperature may be as low as room temperature depending onthe acid and the pressure used. The contacting condition of thehydrolysis reaction may range preferably up to and including about 150°C. depending on the alpha-hydroxysulfonic acid used. In a more preferredcondition the temperature is at least from about 80° C., most preferablyat least about 100° C. In a more preferred condition the temperaturerange up to and including about 90° C. to about 120° C. The reaction ispreferably conducted at as low a pressure as possible, given therequirement of containing the excess sulfur dioxide. The reaction mayalso be conducted at a pressure as low as about 0.1 bara, preferablyfrom about 3 bara, to about pressure of as high as up to 11 bara. Thetemperature and pressure to be optimally utilized will depend on theparticular alpha-hydroxysulfonic acid chosen and optimized based oneconomic considerations of metallurgy and containment vessels aspracticed by those skilled in the art.

Numerous methods have been utilized by those skilled in the art tocircumvent these obstacles to mixing, transport and heat transfer. Thusweight percentage of biomass solids to total liquids (consistency) maybe as low as 1% or higher depending on the apparatus chosen and thenature of the biomass (even as high as 33% if specialized equipment isdeveloped or used). The solids percent is weight percent of dry solidsbasis and the wt % liquids contains the water in the biomass. In apreferred embodiment, where a more conventional equipment is desired,then the consistency is from at least 1 wt %, preferably at least about2 wt %, more preferably at least about 8 wt %, up to about 25 wt %,preferably to about 20 wt %, more preferably to about 15 wt %.

The temperature of the hydrolysis reaction can be chosen so that themaximum amount of extractable carbohydrates are hydrolyzed and extractedas sugar (more preferably pentose and/or hexose) or monosaccharide fromthe biomass feedstock while limiting the formation of degradationproducts. The temperatures required for successful pretreatment arecontrolled by the reaction time, the pH of the solution (acidconcentration), and the reaction temperature. Thus as the acidconcentration is raised, the temperature may be reduced and/or thereaction time extended to accomplish the same objective. The advantagesof lowering the reaction temperature are that the fragile monomericsugars are protected from degradation to dehydrated species such asfurfurals and that the lignin sheath is not dissolved or melted andre-deposited upon the biomass. If high enough levels of acid areemployed, temperatures can be reduced below the point at which sugardegradation or lignin deposition are problematic; this in turn is madepossible through the use of reversible α-hydroxysulfonic acids. In sucha low temperature process it becomes possible to recycle a sugarsmixture from the back of a pretreatment process to the front of apretreatment process. This allows the sugars to build to a high steadystate value while still handling a pumpable slurry through thepretreatment process. In this process biomass, water, andα-hydroxysulfonic acid are combined in an acid hydrolysis step andreacted to effect biomass pretreatment. The acids are separated from thereaction mixture as described above and recycled to the pretreatmentreactor. Then a concentrated high solids/liquid mixture (wet solidstream) is separated from the bulk liquid, which may be recycled to thereactor as well. The aqueous phase from the dehydration step is recycledto the hydrolysis step and in this manner the biomass to liquids ratiois set by the feed ratio of these components and the optimized target ofwet biomass to move into enzymatic hydrolysis and/or acid catalyzeddehydration.

In some embodiments, a plurality of reactor vessels may be used to carryout the hydrolysis reaction. These vessels may have any design capableof carrying out a hydrolysis reaction. Suitable reactor vessel designscan include, but are not limited to, batch, trickle bed, co-current,counter-current, stirred tank, down flow, or fluidized bed reactors.Staging of reactors can be employed to arrive the most economicalsolution. The remaining biomass feedstock solids may then be optionallyseparated from the liquid stream to allow more severe processing of therecalcitrant solids or pass directly within the liquid stream to furtherprocessing that may include enzymatic hydrolysis, fermentation,extraction, distillation and/or hydrogenation. In another embodiment, aseries of reactor vessels may be used with an increasing temperatureprofile so that a desired sugar fraction is extracted in each vessel.The outlet of each vessel can then be cooled prior to combining thestreams, or the streams can be individually fed to the next reaction forconversion.

Suitable reactor designs can include, but are not limited to, abackmixed reactor (e.g., a stirred tank, a bubble column, and/or a jetmixed reactor) may be employed if the viscosity and characteristics ofthe partially digested bio-based feedstock and liquid reaction media issufficient to operate in a regime where bio-based feedstock solids aresuspended in an excess liquid phase (as opposed to a stacked piledigester). It is also conceivable that a trickle bed reactor could beemployed with the biomass present as the stationary phase and a solutionof α-hydroxysulfonic acid passing over the material.

In some embodiments, the reactions described below are carried out inany system of suitable design, including systems comprisingcontinuous-flow (such as CSTR and plug flow reactors), batch, semi-batchor multi-system vessels and reactors and packed-bed flow-throughreactors. For reasons strictly of economic viability, it is preferablethat the invention is practiced using a continuous-flow system atsteady-state equilibrium. In one advantage of the process in contrastwith the dilute acids pretreatment reactions where residual acid is leftin the reaction mixture (<1% wt. sulfuric acid), the lower temperaturesemployed using these acids (5 to 20% wt.) results in substantially lowerpressures in the reactor resulting in potentially less expensiveprocessing systems such as plastic lined reactors, duplex stainlessreactors, for example, such as 2205 type reactors.

The wet solids stream 220 contains at least 5 wt % of undissolved solidscontaining cellulose, preferably in the range of 12 wt % to about 50 wt% undissolved solids containing cellulose, preferably in the range of 15wt % to 35 wt % undissolved solids containing cellulose, and morepreferably in the range of 20 wt % to 30 wt % undissolved solidscontaining cellulose, based on the wet solid product stream.

The bulk liquid stream 210 comprises carbohydrate compounds, inparticular comprises C5-carbohydrates, such as pentose. The bulk liquidstream 210 may optionally comprise C6-carbohydrates such as hexose,however, the majority of the carbohydrates in the bulk liquid stream areC5-carbohydrates, i.e. bulk liquid stream 210 comprises carbohydratecompounds, of which carbohydrate compounds at least 50 wt % areC5-carbohydrate compounds, based on the total weight of the carbohydratecompounds in bulk liquid stream 210. The bulk liquid stream may compriseof up to 20 wt % to 95 wt % of the liquid contained in the digestionproduct stream.

At least a portion of the bulk liquid stream 216 is provided to adehydration system 300 where the stream is passed over a solid aciddehydration catalyst under dehydration reaction conditions, with theaddition of a additional solvent as appropriate. At least a portion ofthe bulk liquid stream may be recycled 218 to the hydrolysis reactionsystem 114, where the bulk liquid stream may be recycled in such amanner as to keep the hydrolysis reaction pumpable along with theaqueous recycle stream 318, preferably about 20 wt % or less of solidscontent in the hydrolysis reactor 114. An advantage of recycling part ofthe bulk liquid stream to the hydrolysis reaction system 114 is that theconcentration of C5-carbohydrates in bulk liquid stream 210 can beincreased while keeping the overall reaction mixture pumpable withoutthe addition of dilution water. Required make-up water can be introducedto the process system in numerous locations as appropriate to achievedesired results.

Dehydration system 300 is a biphasic system for performing a dehydrationreaction. The use of a biphasic system compared to typical aqueouscommercial processes for furfural production has the advantage thatimproved furfural yields may be obtained due to the in-situ extractionof furfural into the organic phase. Furthermore the use of an aqueousand organic phase allows for a more efficient separation of the furfuralfrom the aqueous phase.

Dehydration process stream 300 is then introduced to the extractionsystem (preferably a liquid-liquid extraction system) 330. Aqueousrecycle stream 318 is at least in part recycled to hydrolysis reactionsystem 114. The organic product stream 350 is then introduced to aseparation zone 400, preferably comprising one or more distillationunits, so as to produce the desired product, furfural. Optionally, partof organic product stream 350 may be recycled to dehydration system 300.By recycling part of organic product stream to dehydration system 300,the concentration of furfural in stream 350 may be increased which isbeneficial when separating the furfural form the organic solvent.

The solid acid catalyst used in the dehydration step (“dehydrationacid”) can be a heterogeneous solid acid catalyst as long as it cancatalyze the dehydration of C5 carbohydrates to furfural and/or itsderivatives. Preferred heterogeneous solid acid catalyst, may includeacidic ion exchange resins, for example, such as sulfonated resins(e.g., polystyrene sulfonate), acidic zeolites, for example, such asHZSM-5 HY, HUY and HUSY zeolites (e.g., with 0.5 to 0.74 nm porediameters) β-zeolite (e.g. Si/Al=12), H-beta (e.g., Si/Al=19), layeredclays such as hydroltalcites, amorphous silica alumina, gamma alumina,mesoporous silicate and aluminosilicates such as MCM-41, aluminumincorporated mesoporous silica, for example, such as Al-MCM-41, andAL-SBA-15, sulfonic acid functionalized metal oxides such as sulfatedtin oxides, and microporous silicoaluminophasphates (SAPO),perfluorinated ion-exchange materials such as Nafion® SAC-13 andNafion-117, sulfonated grapheme oxide, and heteropolyacids. Commercialexamples of acidic ion exchange resin catalysts includes, for example,DOWEX™ HCR-S, DOWEX HCR-W2, DOWEX M-31, DOWEX G-26, DOWEX DR 2013,Amberlyst™ 70 (available from Dow Chemical Co.) and Amberlyst® 15 (wet)ion-exchange resin, AMBERJET™ 1200 (H) ion exchange resin, Amberlite®IR-118 (H), Amberlite® IR-120 (plus), Amberlite 15, Amberlite® XN-1010,and Nafion® Resins (available from Signma-aldrichCo.) Commercial exampleof amorphous silica alumina catalyst include, for example X-600 catalyst(available from Criterion Catalysts & Technologies L.P.).

Total acid site strength of the solid acid catalyst is higher than 0.05mmol/g, preferably higher than 0.5 mmol/gm, more preferably higher than2 mmol/g. It can be even be higher than 10 mmol/gm as measured byNH₃-TPD method Preferably, the amount of acid catalyst used can be inthe range of 0.05 mmol/g or 10 mmol/g of total acid sites of solid acidcatalyst. Typically, the acid catalyst will be in excess of the amountnecessary to convert at least 80%, preferably 90% of the xylose underconditions described below.

Since biomass may contains caustic inorganic materials (such as calciumand potassium), we have found that the formation of the anion salt ofα-hydroxysulfonic acid represent the largest “loss” of theα-hydroxysulfonic acid in the reversible acid pretreatment process. Whenα-hydroxysulfonic acid encounters a basic species, such as a carbonate,the anionic salt form of the acid is generated. This acid salt is notreversible as the α-hydroxysulfonic acid must be in the protonic form torevert to primary components.

We have found that by titrating the α-hydroxysulfonic acid salt withstrong acid and then reverting the α-hydroxysulfonic acid as its primarycomponents, the acid components can be recovered virtuallyquantitatively providing for a cost reduction in the reversible acidpretreatment process. Thus if maximum recovery of the α-hydroxysulfonicacid is desired, a strong acid, such as mineral acid is optionally addedin small quantity sufficient to titrate the salt.

By optionally adding about a molar equivalent amount of a mineral acid(such as for example, hydrochloric, sulfuric or phosphoric acid) to asolution of salts of α-hydroxysulfonic acids, an equilibrium can beachieved between the protonic and mineral salt versions of the acids. Bythe term about a molar equivalent, the molar equivalent may be ±20%.

For example, when the potassium salt of alpha-hydroxyethanesulfonic acid(HESA) is treated with an equivalent of sulfuric (a divalent acid),phosphoric (a divalent strong acid), or hydrochloric acid (a monovalentacid), the HESA can be flashed overhead as SO₂ and acetaldehyde leavingpotassium sulfate, potassium monohydrogen phosphate, or potassiumchloride in solution. When HESA is recovered overhead, the pH of thesalt solution rises to what it was prior to the addition of the mineralacid.

The reaction (titration) of the α-hydroxysulfonic acid salt with strongmineral acid and then reverting the α-hydroxysulfonic acid as itsprimary components is illustrated in equation 3 for calcium salt ofα-hydroxysulfonic acid.

By adding about a molar equivalent amount of a mineral acid (e.g.,hydrochloric, sulfuric or phosphoric acid) to a solution of salts ofα-hydroxysulfonic acids, equilibrium can be achieved between theprotonic and mineral salt versions of the acids. As only theα-hydroxysulfonic acid is reversible to volatile components, followingLe Chatelier's principle, all of the alpha-hydroxysulfonic acid can berecovered and the salt of the mineral acid is formed.

The titration may be carried out in 300, or carried out in 114-120. Forexample, it may be preferable to carry out the titration in 114-120 forreduction of the amount of salt trapped in the wet solid residue 220.

The second product stream 126 is transferred to a separation system 200(solid-liquid separator or phase separator), where the wet solids stream220 comprising solids, and primarily solids comprising cellulose, isseparated from the bulk liquid stream 210 that contains primarilyC5-carbohydrate products, such as xylose. Examples of suitableseparation method, for example, may include centrifugal force,filtration, decantation, and other like methods. Optionally, at least aportion of the liquid stream containing the residual α-hydroxysulfonicacid from the reaction stream 116 (carbohydrate containing productstream) may be recycled to the hydrolysis reaction system.

At least a first portion of the bulk liquid stream is subsequentlyprovided to a dehydration step for dehydration of the C5-carbohydratesin the bulk liquid product stream, by feeding stream 216 into a reactionvessel of the dehydration step 300.

Either one or both of streams 210 or 216 may be flashed to remove partof the water (not shown) to concentrate streams 210 and/or 216. Theseparation step 200 may be carried out in any suitable solid/liquidseparating device such as, but not limited to, filters, centrifuges,screw presses, etc. As mentioned before, the liquid stream mayoptionally be recycled to the hydrolysis step to build the concentrationof C5-carbohydrates. Optionally, stream 216 may also be subjected to aflash, distillation or multi-effect evaporator to increase theC5-carbohydrate concentration.

The dehydration step 300 occurs in a biphasic mixture of aqueous andorganic phases, the aqueous phase being that carried through fromseparation step 200, the organic phase being one or more organicsolvents that are substantially immiscible with the aqueous phase. Theuse of organic solvent with preferred selectivity towards furfuralextraction, extracts furfural from the aqueous phase as it is formedduring the dehydration reaction. This may improve overall furfuralyield. A further advantage is that by extracting the furfural into theorganic phase, the undesired loss of furfural via degradation reactionshappening in the aqueous phase is reduced.

The preferred organic phase for use in the present invention comprises awater-immiscible organic solvent that is substantially immiscible withthe aqueous phase containing C5-carbohydrate products. Preferably suchwater-immiscible organic solvents have a maximum water solubility ofless than about 30 wt %, preferably less than about 10 wt %, and mostpreferably less than about 2 wt % at ambient (room) temperature. Thepreferred organic solvents are 1-butanol, sec-butyl phenol (SBP), MIRK,toluene and dichloromethane (DCM). Other organic phases, especiallyother alcohols, ketones, and halogenated alkanes, may also be utilized.Thus, for example, organic solvents such as straight or branchedalcohols (e.g. pentanol, tertbutyl alcohol, etc.), cyclic alcohols(e.g., cyclohexanol), straight or branched alkanones (e.g. butanone(i.e., methylethyl ketone (MEK)), pentanone, hexanone, heptanone,diisobutylketone, 3-methyl-2-butanone, 5-methyl-3-heptanone, etc.), andcycloalkanones (e.g., cyclobutanone, cyclopentanone, cyclohexanone,etc.) may be used in the present invention. Aliphatic and cycloaliphaticethers (e.g., dichloroethylether, dimethyl ether, MeTHF), saturated andunsaturated aliphatic or aromatic hydrocarbons (decane, toluene,benzene, cymene, 1-methyl naphthalene), oxygenated hydrocarbons (e.g.furan, nonyl phenol, etc.), and nitroalkanes (e.g., nitromethane,nitropropane, etc.) may also be used. Likewise, halogenated derivativesof the above-noted compounds, as well as other halogenated alkanes mayalso be used as the organic phase (e.g., chloromethane,trichloromethane, trichloroethane, and the like). Lignin derivedsolvents such as Guaiacol, Eugenol, 2-Methoxy-4-propylphenol (MPP),2-Methoxy-4MethylPhenol (MMP) or mixture thereof may also be used.Combination of solvents may also be used to fine tune the extractingcapability of the solvent.

Preferably, the organic solvent or the combination of organic solventscan extract 80 mol % or more of the furfural produced from the aqueousphase, while at the same time dissolve less than 1 wt %, even preferablyless than 0.1 wt %, still more preferably less than 0.01 w %. of water.

The weight percentage of organic phase material is in a range suitableto create a biphasic system with the aqueous phase, e.g., from about 5%by weight to about 95% by weight, based on the combined weight of theaqueous phase and organic phase.

The dehydration process step 300 is carried out for a period of time(residence time) ranging from about 1 minute to about 24 hrs, preferablyfor a period of time ranging of from about 5 minutes to about 12 hrs,more preferably from about 10 minutes to about 6 hours, still morepreferably 30 minutes to 4 hrs., even still more preferably 30 minutesto 2 hrs. or for times within these ranges, at an elevated temperatureabove about 100° C., including in the range from about 100° C. to about250° C., from about 110° C. to 200° C. and from about 140° C. to about180° C. One or more dehydration acids as described above may be used inthe dehydration step in order to catalyze the reaction process. Thepressure is preferably autogenous pressure of hot steam.

The concentration of the C5-carbohydrate compounds in the dehydrationreactor 300 can vary depending upon the product to be obtained. However,in accordance with aspects of the present invention, it has been foundthat the reaction is optimized for obtaining furfural or other furfuralderivatives when the concentration of C5 components during thedehydration process step 300 is between about 0.1 wt % and 20 wt %, morepreferably between about 0.2 wt % and 10 wt %, inclusive %, based on theweight of the aqueous phase.

During the dehydration process step, at least part, and preferablysubstantially all, of the C5-carbohydrate compounds are converted tofurfural. Optionally, other furfural derivatives may also be formed. Dueto the nature of the furfural, and optional other furfural derivatives,the furfural preferably reside in the organic phase of the biphasicmixture. Due to the preference of the formed furfural to reside in theorganic phase in rather than in the aqueous phase at least part of theformed furfural, and preferably more than 50 wt %, still more preferably75 wt % of the formed furfural will dissolve in the organic phase.

Following the dehydration step 300, dehydration product stream 310 istransferred to an extractor (preferably liquid-liquid extractor) for theextraction step 330, optionally after cooling of the stream. Thedehydration product comprises at least part of the biphasic mixture,comprising an aqueous phase and a water-immiscible organic phase, whichwas present in the reaction vessel during the dehydration process andthus comprises water, organic solvent and further comprises furfuralthat was formed by the dehydration of the C5-carbohydrates. Thefurfural, herein will be predominantly dissolved in the organic solvent.

The extraction 330 can be operated at a temperature range from aboutroom temperature to about the dehydration temperature, so long as theliquid separates into two liquid phases at the extractor temperature.The organic phase is separated from the aqueous phase, and thus obtainedaqueous recycle stream 318 may be fed directly back into the processloop to the hydrolysis reaction step. Depending upon the salt, andoptional other organic byproduct, content of the aqueous stream, aqueousrecycle stream 318 may be treated to remove unwanted or excessiveamounts of salts and/or organic byproducts. Preferably, aqueous recyclestream is subjected to a separation step (not shown). The recoveredaqueous recycle stream obtained after treatment of aqueous recyclestream, is reintroduced to the hydrolysis reaction step 114. Salts, andoptionally other organic byproducts like humins, are formed as abyproduct during one or more of the process steps. Typically, part ofstream 318 may also be purged from the process to prevent the build-upof byproducts as part of separation step.

Prior to undergoing the liquid-liquid extraction step, dehydrationproduct stream 330 may optionally be routed through a, preferablysolid/liquid, separation step, to remove any insoluble humins or othertar that may have been formed during the dehydration step 300, and whichmay otherwise negatively interfere with the separation of the organicphase from the aqueous phase, or later separation or purification steps(not shown). The humins or tar will predominantly end up in the solidphase and will thus not, or to a lesser extent, affect the subsequentorganic/aqueous separation step 330. Formation of tar, char, and/orhumins is a well known problem associated with the production ofbio-based products, and their non removal from the production stream canresult in problems during downstream purification and/or separationsteps.

The organic phase, i.e. the organic solvent, is retrieved fromextraction step 330 as organic product stream 350, containing the targetorganic compounds such as furfural and furfural derivatives. Although,part of organic product stream may be recycled to dehydration reactor300, the majority of organic product stream 350 is subjected to aseparation step, preferably one or more distillation steps, inseparation zone 400. Residual water from the reaction that was notremoved during the liquid-liquid extraction step, and which may containacetic acid or other water-soluble impurities, is removed via flowstream from separation zone 400, with recovery of furfural via stream420.

Organic solvents 410 removed/recovered during the separation inseparation zone 400 step can be recycled back into the process, such asby reintroduction back into the dehydration reaction vessel 300, inorder to minimize production costs and maintain the reaction process andprocess efficiency. Alternatively, at least part of the organic solventstream 410 can be directed to a further solvent purification processsuch as column distillation/separation or solvent-solvent extraction(not shown), prior to reintroduction back into the production process,so as to remove impurities, primarily humins (heavy byproducts), as wellas purify the solvent before reintroduction. After the solventpurification step, fresh solvent may be added to the purified solventstream prior to reintroduction to the dehydration reaction vessel so asto maintain the required volume of organic phase in the dehydrationstep.

Wet solids stream 220 may still contain substantial amounts of residualC5-carbohydrates. In order to extract any residual C5 carbohydrates, thewet solids stream are preferably, washed with at least part of aqueousstream 318 (not shown) prior to providing the aqueous stream to thehydrolysis system 114.

In a particular embodiment of the process according to the invention thewet solids stream 220 may be further treated to produce alcohols andglycols. The solids comprising cellulose contained in wet solids stream220, once separated from the C5-carbohydrate-containing liquid processstream 210 as discussed in detail above, can be subjected to a varietyof processes. It is contemplated that the wet solids containingcellulose in the wet solids stream 220 (and products separatedtherefrom) can be separated out as pulp for use in the paper productindustry, and can also be used to generate biomass-derived alcohols,biomass derived mono- and diacids, biomass-derived (polymeric) polyols,biomass-derived diols, power, and other chemicals useful in industrialmanufacturing operations. As explained in more detail herein below, thesolids containing cellulose may be used to from alcohols such asbutanol/ethanol or butanediol, e.g. via hydrolysis and fermentation.Glycols like ethylene glycol and propylene glycol may be produced viahydrolysis of the C6 sugars, but may alternatively be produced by acatalytic conversion of the C6 sugars to diols. The cellulose can alsobe converted to mono- and diacids such as acetic acid, lactic acid,levulinic acid or succinic acid by means of fermentation or chemicalconversion.

The solids may also be used to generate power by burning the wet solidresidue e.g. in a in co-generation boiler. Alternatively, the wet solidproduct stream may be converted and optionally dried to form pellets,which can be used to produce for instance power at remote locations.

Exemplary biomass-derived diols include, but are not limited to, C₂-C₁₀diols such as ethylene glycol, propylene glycol, 1,4-butane diol (BDO),pentane diol, propylene glycol, 1,2-propanediol, 1,3-propanediol,1,5-pentanediol, 1,4-pentanediol, 1,2-butanediol, 1,3-butanediol,2,3-butanediol, 1,4-butanediol 1,2-pentanediol, 1,3-pentanediol,1,4-pentanediol, 1,5-pentanediol, 2,3-pentanediol, 2,4-pentanediol, andcombinations thereof.

Exemplary chemicals that can be produced from the production stepsdetailed herein include butanol (both n-butanol and iso-butanol),butanol mixes, HMF (hydroxymethyl)furfural and MMF (5-methoxymethylfurfural).

Additionally, the solids removed during various steps of the processdescribed herein may be converted to power or energy, such as by burningor otherwise treating the solids in a power plant or similar powerproduction facility, the power being storable for later sale, or used tofuel the closed-loop process, thereby increasing the process efficiency.The solid tar and/or humins can also be converted to a fuel gas, such asby gasification methods to produce low tar fuel gas with low emissionsand no toxic waste streams or burned as fuel in a boiler.

The residual α-hydroxysulfonic acid can be removed by application ofheat and/or vacuum from carbohydrate containing product stream toreverse the formation of α-hydroxysulfonic acid to its starting materialto produce a stream containing fermentable sugar substantially free ofthe α-hydroxysulfonic acid. In particular, the product stream issubstantially free of α-hydroxysulfonic acid, meaning no more than about2 wt % is present in the product stream, preferably no more than about 1wt %, more preferably no more than about 0.2 wt %, most preferably nomore than about 0.1 wt % present in the product stream. The temperatureand pressure will depend on the particular α-hydroxysulfonic acid usedand minimization of temperatures employed are desirable to preserve thesugars obtain in treatment reactions. Typically the removal may beconducted at temperatures in the range from about 50° C., preferablyfrom about 80° C., more preferably from 90° C., to about 110° C., up toabout 150° C. The pressure should be such that the α-hydroxysulfonicacid is flashed in its component form at the temperature for removal ofthe acid. This pressure should be at or above the pressure of thesaturated steam at such temperature but low enough to flash theα-hydroxysulfonic acid in its component form. For example, the pressuremay be in the range of from about 0.1 bara, to about 5 bara, morepreferably from 0.5 bara to about 2 bara. It can be appreciated by aperson skill in the art that the treatment reaction 114 and the removalof the acid 120 can occurred in the same vessel or a different vessel orin a number of different types of vessels depending on the reactorconfiguration and staging as long as the system is designed so that thereaction is conducted under condition favorable for the formation andmaintenance of the alpha-hydroxysulfonic acid and removal favorable forthe reverse reaction (as components). As an example, the reaction in thereactor vessel 114 can be operated at approximately 100° C. and apressure of 3 bara in the presence of alpha-hydroxyethanesulfonic acidand the removal vessel 120 can be operated at approximately 110° C. anda pressure of 0.5 bara. It is further contemplated that the reversioncan be favored by the reactive distillation of the formedalpha-hydroxysulfonic acid. In the recycling of the removed acid,optionally additional carbonyl compounds, SO₂, and water may be added asnecessary. The removed starting material and/or alpha-hydroxysulfonicacid may be condensed and/or scrubbed by contact with water and recycledto the reaction system114 as components or in its recombined form.

The preferable residence time of the biomass to contact with theα-hydroxysulfonic acid in the hydrolysis reaction system may be in therange of about 5 minutes to about 4 hours, most preferably about 15minutes to about 1 hour.

In one embodiment, the cellulose containing product stream can furtherbe hydrolyzed by other methods, for example by enzymes to furtherhydrolyze the biomass to sugar products containing pentose and hexose(e.g., glucose) and fermented to produce alcohols such as disclosed inUS Publication No. 2009/0061490 and U.S. Pat. No. 7,781,191, whichdisclosures are hereby incorporated by reference.

The term “fermentable sugar” refers to oligosaccharides andmonosaccharides that can be used as a carbon source (e.g., pentoses andhexoses) by a microorganism in a fermentation process. In an enzymatichydrolysis-fermentation processes the pH of the wet solids stream may beadjusted so that it is within a range which is optimal for the cellulaseenzymes used. Generally, the pH of the pretreated feedstock is adjustedto within a range of about 3.0 to about 7.0, or any pH there between.

The temperature of the treated feedstock is adjusted so that it iswithin the optimum range for the activity of the cellulase enzymes.Generally, a temperature of about 15° C. to about 100° C., about 20° C.to about 85° C., about 30° C. to about 70° C. preferably or anytemperature there between, is suitable for most cellulase enzymes. Thecellulases, β-glucosidase and other accessory enzymes required forcellulose hydrolysis are added to the pretreated feedstock, prior to,during, or after the adjustment of the temperature and pH of the aqueousslurry after pretreatment. Preferably the enzymes are added to thepretreated lignocellulosic feedstock after the adjustment of thetemperature and pH of the slurry.

By the term “cellulase enzymes” or “cellulases,” it is meant a mixtureof enzymes that hydrolyze cellulose. The mixture may includecellobiohydrolases (CBH), glucobiohydrolases (GBH), endoglucanases (EG),glycosyl hydrolyase family 61 proteins (GH61) and β-glucosidase. By theterm “β-glucosidase”, it is meant any enzyme that hydrolyzes the glucosedimer, cellobiose, to glucose. In a non-limiting example, a cellulasemixture may include EG, CBH, GH61 and β-glucosidase enzymes.

The enzymatic hydrolysis may also be carried out in the presence of oneor more xylanase enzymes. Examples of xylanase enzymes that may also beused for this purpose and include, for examples, xylanase 1, 2 (Xyn1 andXyn2) and β-xylosidase, which are typically present in cellulasemixtures.

The process can be carried out with any type of cellulase enzymes,regardless of their source. Non-limiting examples of cellulases whichmay be used include those obtained from fungi of the genera Aspergillus,Humicola, and Trichoderma, Myceliophthora, Chrysosporium and frombacteria of the genera Bacillus, Thermobifida and Thermotoga. In someembodiments, the filamentous fungal host cell is an Acremonium,Aspergillus, Aureobasidium, Bjerkandera, Ceriporiopsis, Chrysosporium,Coprinus, Coriolus, Cryptococcus, Filibasidium, Fusarium, Humicola,Magnaporthe, Mucor, Myceliophthora, Neocallimastix, Neurospora,Paecilomyces, Penicillium, Phanerochaete, Phlebia, Piromyces, Pleurotus,Schizophyllum, Talaromyces, Thermoascus, Thielavia, Tolypocladium,Trametes, or Trichoderma cell.

The cellulase enzyme dosage is chosen to convert the cellulose of thepretreated feedstock to glucose. For example, an appropriate cellulasedosage can be about 1 to about 100 mg enzyme (dry weight) per gram ofcellulose.

In practice, the hydrolysis may carried out in a hydrolysis system,which may include a series of hydrolysis reactors. The number ofhydrolysis reactors in the system depends on the cost of the reactors,the volume of the aqueous slurry, and other factors. The enzymatichydrolysis with cellulase enzymes produces an aqueous sugar stream(hydrolyzate) comprising glucose, unconverted cellulose, lignin andother sugar components. The hydrolysis may be carried out in two stages(see U.S. Pat. No. 5,536,325, which is incorporated herein byreference), or may be performed in a single stage.

In the fermentation system, the aqueous sugar stream is then fermentedby one or more than one fermentation microorganism to produce afermentation broth comprising the alcohol fermentation product useful asbiofuels. In the fermentation system, any one of a number of knownmicroorganisms (for example, yeasts or bacteria) may be used to convertsugar to ethanol or other alcohol fermentation products. Themicroorganisms convert sugars, including, but not limited to glucose,mannose and galactose present in the clarified sugar solution to afermentation product.

Many known microorganisms can be used in the present process to producethe desired alcohol for use in biofuels. Clostridia, Escherichia coli(E. coli) and recombinant strains of E. coli, genetically modifiedstrain of Zymomonas mobilis such as described in US2003/0162271, U.S.Pat. No. 7,741,119 and U.S. Pat. No. 7,741,084 (which disclosures areherein incorporated by reference) are some examples of such bacteria.The microorganisms may further be a yeast or a filamentous fungus of agenus Saccharomyces, Kluyveromyces, Candida, Pichia,Schizosaccharomyces, Hansenula, Kloeckera, Schwanniomyces, Yarrowia,Aspergillus, Trichoderma, Humicola, Acremonium, Fusarium, andPenicillium. The fermentation may also be performed with recombinantyeast engineered to ferment both hexose and pentose sugars to ethanol.Recombinant yeasts that can ferment one or both of the pentose sugarsxylose and arabinose to ethanol are described in U.S. Pat. No.5,789,210, U.S. Pat. No. 6,475,768, European Patent EP 1727890, EuropeanPatent EPI 863,901 and WO 2006/096130 which disclosures are hereinincorporated by reference. Xylose utilization can be mediated by thexylose reductase/xylitol dehydrogenase pathway (for example, WO9742307A1 19971113 and WO9513362 A1 19950518) or the xylose isomerase pathway(for example, WO2007028811 or WO2009109631). It is also contemplatedthat the fermentation organism may also produce fatty alcohols, forexample, as described in WO 2008/119082 and PCT/US07/011923 whichdisclosure is herein incorporated by reference. In another embodiment,the fermentation may be performed by yeast capable of fermentingpredominantly C6 sugars for example by using commercially availablestrains such as Thermosacc and Superstart.

Preferably, the fermentation is performed at or near the temperature andpH optima of the fermentation microorganism. For example, thetemperature may be from about 25° to about 55° C., or any amount therebetween. The dose of the fermentation microorganism will depend on otherfactors, such as the activity of the fermentation microorganism, thedesired fermentation time, the volume of the reactor and otherparameters. It will be appreciated that these parameters may be adjustedas desired by one of skill in the art to achieve optimal fermentationconditions.

The fermentation may be conducted in batch, continuous or fed-batchmodes, with or without agitation. The fermentation system may employ aseries of fermentation reactors.

In some embodiment, the hydrolysis system and fermentation system may beconducted in the same vessel. In one embodiment, the hydrolysis can bepartially completed and the partially hydrolyzed stream may befermented. In one embodiment, a simultaneous saccharification andfermentation (SSF) process where hydrolysis system may be run until thefinal percent solids target is met and then the hydrolyzed biomass maybe transferred to a fermentation system.

The fermentation system produces an alcohol stream preferably containingat least one alcohol having 2 to 18 carbon atoms. In the recoverysystem, when the product to be recovered in the alcohol stream is adistillable alcohol, such as ethanol, the alcohol can be recovered bydistillation in a manner known to separate such alcohol from an aqueousstream. If the product to be recovered in the alcohol stream is not adistillable alcohol, such as fatty alcohols, the alcohol can berecovered by removal of alcohols as solids or as oils from thefermentation vessel, thus separating from the aqueous effluent stream.While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexamples herein described in detail. It should be understood, that thedetailed description thereto are not intended to limit the invention tothe particular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims. The present invention will be illustrated by the followingillustrative embodiment, which is provided for illustration only and isnot to be construed as limiting the claimed invention in any way.

ILLUSTRATIVE EMBODIMENTS General Methods and Materials

In the examples, the aldehyde or aldehyde precursors were obtained fromSigma-Aldrich Co. α-hydroxyethane sulfonic acid (HESA) was preparedaccording to US2012/0122152.

Biphasic Dehydration

Biphasic acid dehydration of C5 carbohydrates (primarily xylose) inaqueous streams for examples 3-4 were carried out in a 300 ml Parr batchreactor (Parr Instruments, Inc.). In a typical run, 3 gm of solid acidcatalyst is added to a mixture of 100 gm of 5 wt % xylose aqueoussolution and 100 gm of an immisicible organic solvent such as Toluene.The reactor was then heated to the reaction temperature of 140° C. andheld at that temperature for a residence time of 10 h. After thereaction is complete the reaction mixture were weighed and transferredinto a separatory funnel to allow for two liquid phases to separate.After separation, each layer was weighed and analyzed for its content.Aqueous layer was analyzed using HPLC and the Organic layer was analyzedusing GC as described below.

Analytical Methods

The aqueous layers from the acid dehydration runs were analyzed andquantified for various components such as glucose, xylose, arabinose,mannose, formic acid, acetic acid, levulinic acid, furfural usinghigh-performance liquid chromatography (HPLC) system (Shimadzu) equippedwith a refractive index detector (Shimadzu) on a BIO-RAD 87H Column.Prior to injection, the samples were filtered through 0.45 μm HV filters(Millipore, Bedford, Mass., USA), and a volume of 10 μL was injected.The mobile phase for the column was 5 mM H₂SO₄ in Milli-Q water at aflow rate of 0.6 mL/min.

In a typical biphasic dehydration run the furfural concentration in theorganic phase or layer was measure using GC. Agilent 6890 GC with aDB-1301 capillary column installed in its split/splitless inlet was usedwith the FID. The column parameters were 30 m length, 0.25 mm ID, and1.0 μm film thickness. Method parameters were as follows:

Oven Temp Program—40 C Hold 3 min, Ramp 10 C/min to 280 C Hold 3 min

Inlet Temp 250 C, Injection Volume 1.0 μl, Split ratio 100:1, ConstantPressure 20 psi Helium Carrier gas

Detector Temp 325 C, H₂ flow 35 ml/min, Air 400 ml/min, and HeliumMakeup 25 ml/min

Calculations

Xylose Conversion={[mole of Xylose]_(feed)−[mole of Xylose]_(AL)}/[moleof Xylose]_(feed)

Furfural Selectivity={[moles of FUR]_(AL)+[moles of FUR]_(OL)}/{[mole ofXylose]_(feed)−[mole of Xylose]_(AL)}

Furfural yield=Xylose Conversion*Furfural Selectivity

Where FUR=Furfural, AL=Aqueous layer or phase and OL=organic layer orphase.

Example 1: Hydrolysis with α-Hydroxyethane Sulfonic Acid (HESA)—Stream 1

Into a 1 gallon C276 Parr reactor fitted with in situ IR optics wasadded approximately 350 grams of compositionally characterized cornstover [dry basis: xylan 24% wt.; glucan 33 wt., 16% w moisture] choppedto nominal 0.5 cm particles. To this was added approximately 2600 grams(runs 1-3) and 2200 g (runs 4-6) of 5% wt. α-hydroxyethane sulfonic acid(HESA) prepared by the dilution of a 40% wt. stock solution of the acid,acid recycled from vaporization of components at the end of a reactioncycle, excessive pressate liquid from the bottoms after pressing theun-dissolved to about 20-22% w. Runs 1-3 targeted about 11% w fresh drycorn stover to begin a run, while runs 4-6 targeted about 13% w. Targetconcentration of acid was confirmed by proton NMR of the startingmixture integrating over the peaks for water and the acid. The reactortop with a 4 blade down pitch impeller was placed on top of the reactionvessel and the reactor sealed. The pressure integrity of the reactorsystem and air atmosphere replacement was accomplished by pressurizationwith nitrogen to 100 psig where the sealed reactor was held for 15minutes without loss of pressure followed by venting to atmosphericpressure. IR acquisition was initiated and the reaction mixture stirredat 500 rpm. The reactor was then heated to 120° C. and held at targettemperature for 60 minutes. During this period of time the in situ IRreveals the presence of HESA, SO₂, and acetaldehyde in an equilibriummixture. An increase in sugars is evident in the IR spectra, with anincrease in the band height typical of xylose and glucose beingapparent. At the end of the reaction period the acid reversal wasaccomplished via opening the gas cap of the reactor to an overheadcondensation system for recovery of the acid and simultaneouslyadjusting the reactor temperature set point to 100° C. Vaporization fromthe reactor quickly cools the reactor contents to the 100° C. set point.The overhead condensation system was comprised of a 1 liter jacketedflask fitted with a fiber optic based in situ IR probe, a dry iceacetone condenser on the outlet and the gas inlet arriving through an18″ long steel condenser made from a core of ¼″ diameter C-276 tubingfitted inside of ½″ stainless steel tubing with appropriate connectionsto achieve a shell-in-tube condenser draining downward into the recoveryflask. The recovery flask was charged with approximately 400 grams of DIwater and the condenser and jacketed flask cooled with a circulatingfluid held at 1° C. The progress of the acid reversion was monitored viathe use of in situ IR in both the Parr reactor and the overheadcondensation flask. During the reversal the first component to leave theParr reactor was SO₂ followed quickly by a decrease in the bands forHESA. Correspondingly the bands for SO₂ rise in the recovery flask andthen quickly fall as HESA was formed from the combination of vaporizedacetaldehyde with this component. The reversal was continued until thein situ IR of the Parr reactor showed no remaining traces of theα-hydroxyethane sulfonic acid. The IR of the overheads revealed that theconcentration of the HESA at this point had reached a maximum and thenstarted to decrease due to dilution with condensed water, free ofα-hydroxyethane sulfonic acid components, building in the recoveryflask. The reaction mixture was then cooled to room temperature, openedand the contents filtered through a Buchner funnel with medium filterpaper using a vacuum aspirator to draw the liquid through the funnel.The wet solids are transferred from the Buchner funnel and placed in afilter press where an additional portion of liquid is pressed from thesolids to create a high consistency biomass (about 22% w un-dissolvedsolids) mixture. The dry weight of solid is determined by washing aportion of the solids with water and then oven drying to a constantweight, A small portion of the combined liquid filtrate and pressate isremoved for analysis by HPLC, NMR, and elemental analysis via XRF; theremainder is reserved for the next cycle with fresh biomass. A recycleexperiment is accomplished by combining the primary filtrate and thepressate liquids with a sufficient quantity of HESA, either recycledfrom the overheads of the previous run or fresh acid from a 40% wt.stock solution, and water to yield 2200 to 2600 grams of a 5% wt. acidsolution which are returned to a 1 gallon C276 Parr reactor where it ismixed with another 350 gram portion of fresh biomass. The pretreatmentcycle, venting and recovery, and filtration were repeated five times inaddition to the initial starting run to produce the sample used infurther experimentation. The HPLC analysis of the pressate is givenbelow in Stream 1 (Table 1).

Example 2: Hydrolysis with α-Hydroxyethane Sulfonic Acid (HESA)—Stream 2and 3

Into a 7 gallon 316 stainless steel batch circulating digesterapproximately 1820 grams (29.14% w moisture) of compositionallycharacterized corn stover [dry basis: xylan 17.7% wt.; glucan 33% wt.]chopped to nominal 2 inch particles. A target fresh dry solids toliquids ratio being 9:1 being targeted for each run. The material wasplaced in a basket and is fixed during the run while liquid iscirculated. The solids are removed at the end of the run after a freeliquid drain and pressed to remove additional liquid. 1820 g of freshstover (1290.5 g dry), 1452 g of 40% w of α-hydroxyethane sulfonic acid(HESA) stock solution, 2984 g make-up water, and 7549 g of recyclepressate (make-up water on run 1). The reactor was brought to 120° C. inabout 10 minutes and held for 1 hour. The reactor was then vented toremove the bulk of the acid into a caustic scrubber. The acid was notrecycled for this study and was made up from the stock solution for eachrun. Two streams (Stream 2 and 3) generated by this procedure withdifferent xylose concentration were produced and analyzed as given inTable 1.

TABLE 1 Composition of three streams produced using pressure reversibleacid digestion of biomass Stream 1 Stream 2 Stream 3 Cellobiose % n/dn/d n/d Sucrose % n/d n/d n/d Glucose % 1.366 0.511 1.105 Xylose % 9.2102.925 5.800 Fructose % n/d 0.476 1.013 Arabinose % 1.371 0.037 0.070Formic % 0.130 0.383 0.670 Acetic % 1.174 0.008 0.016 HMF % 0.014 0.0530.093 Furfural % 0.149 0.003 0.003

Example 3: Xylose Acid Dehydration Using Ion Exchange Resin

This acid dehydration run was carried out by adding about 3 gm ofAmberlyst® 15 (wet) ion exchange resin catalyst (from Sigma Aldrich) ina 300 ml parr batch reactor as described above to the biphasic mixture.The reaction temperature was 140° C. with a residence time of 10 h.After the analysis of aqueous and organic layers, the xylose conversionwas 68% and Furfural selectivity was 47%.

Example 4: Xylose Acid Dehydration Using Amorphous Silica Alumina

This acid dehydration run was carried out by adding about 3 gm ofAmorphous SilicaAlumino (ASA) X600 (from CRITERION CATALYSTS &TECHNOLOGIES L.P.) in a 300 ml parr batch reactor as described above tothe biphasic mixture. The reaction temperature was 140° C. with aresidence time of 10 h. After the analysis of aqueous and organiclayers, the xylose conversion was 97% and Furfural selectivity was 37%.

We claim:
 1. A method for producing furfural from biomass materialcontaining pentosan: (a) providing a biomass containing pentosan; (b)contacting the biomass with a solution containing at least oneα-hydroxysulfonic acid thereby hydrolyzing the biomass to produce aproduct stream containing at least one C₅-carbohydrate compound inmonomeric and/or oligomeric form, and α-hydroxysulfonic acid; (c)separating at least a portion of the α-hydroxysulfonic acid from theproduct stream containing at least one C₅-carbohydrate compound toprovide an acid-removed product stream containing the at least oneC₅-carbohydrate compound and recovering the α-hydroxysulfonic acid inits component form; (d) separating a liquid stream containing said atleast one C₅-carbohydrate compound and a wet solid stream containingremaining biomass from the acid-removed product; (e) dehydrating theC₅-carbohydrate compound in at least a first portion of the liquidstream in the presence of a heterogenous solid acid catalyst, in abiphasic reaction medium comprising an aqueous phase and awater-immiscible organic phase, at a temperature in the range of fromabout 100° C. to about 250° C.; (f) separating an organic phase streamcontaining furfural and an aqueous stream from the dehydration productstream; (g) recycling at least a portion of the aqueous stream or asecond portion of the liquid stream to step (b); (h) recovering furfuralfrom the organic phase stream.
 2. The method of claim 1 wherein thesolid acid catalyst is a solid acid catalyst having total acid sitestrength higher than 0.05 mmol/g.
 3. The method of claim 1 wherein thedehydration step is carried out at a temperature in the range of fromabout 140° C. to about 250° C.
 4. The method of claim 1 wherein at leasta second portion of the liquid stream from step (d) is recycled to step(b).
 5. The method of claim 1 wherein at least a portion of the aqueousstream from step (f) is recycled to step (b).
 6. The method of claim 1wherein at least a portion of the aqueous stream from step (f) and asecond portion of the liquid stream from step (d) are recycled to step(b).
 7. The method of claim 1 wherein at least a portion of the organicphase stream after recovery of furfural is recycled to step (e) toprovide the biphasic reaction medium.
 8. The method of claim 7 whereinthe organic phase stream after recovery of furfural is purified prior torecycling to step (e).
 9. The method of claim 2 wherein the contact timeof the first portion of the liquid stream to the heterogenous solidcatalyst is in the range of 1 minute to 24 hours.
 10. The method ofclaim 1 wherein step (b) is carried out at a temperature within therange of about 50° C. to about 150° C. and a pressure within the rangeof 0.1 bara to about 11 bara.
 11. The method of claim 1 wherein thesolid acid catalyst is selected from the group consisting of acidic ionexchange resins, acidic zeolites, layered clay, amorphous silicaalumina, gamma alumina, mesoporous silicate and aluminosilicates,aluminum incorporated mesoporous silica, sulfonic acid functionalizedmetal oxides, and microporous silicoaluminophasphates, perfluorinatedion-exchange materials, sulfonated grapheme oxide, and heteropolyacids.12. The method of claim 11 wherein the solid acid catalyst is selectedfrom the group consisting of acidic ion exchange resins, acidiczeolites, amorphous silica alumina, and mesoporous silicate andaluminosilicates.
 13. The method of claim 12 wherein the solid acidcatalyst is acidic ion exchange resin.
 14. The process of claim 1,wherein the liquid stream separated from the wet solid stream comprisesC5 carbohydrates in a concentration ranging from about 0.1 wt % to about15 wt %.
 15. The method of claim 1 wherein mineral acid is added to step(b) or step (c).
 16. The method of claim 15 wherein recoveringα-hydroxysulfonic acid from a salt of α-hydroxysulfonic acid formed instep (b) in its component form.
 17. The method of claim 7 wherein theseparation in step (c) is carried out at a temperature within the rangefrom about 50° C. to about 150° C. and a pressure within the range fromabout 0.1 bara to about 5 bara.
 18. The method of claim 1 wherein atleast a portion of the aqueous stream from step (f) is contacted withthe wet solid stream prior to recycling to step (b).
 19. The method ofclaim 1 wherein the α-hydroxysulfonic acid is present in an amount offrom about 1% wt. to about 55% wt., based on the solution.
 20. Themethod of claim 1 wherein the α-hydroxysulfonic acid is produced from(a) a carbonyl compound or a precursor to a carbonyl compound with (b)sulfur dioxide or a precursor to sulfur dioxide and (c) water.
 21. Themethod of claim 1 wherein the α-hydroxysulfonic acid is in-situgenerated.
 22. The method of claim 1 wherein recycling at least aportion of the aqueous stream and a second portion of the liquid streamto step (b).